With the increase in population, rapid industrial development and environmental changes, the world is facing a serious shortage of freshwater resources. More and more countries have begun to develop seawater desalination technology to cope with this global crisis. The technologies commonly used in seawater desalination at present include multi-stage flash distillation (MSF), low-temperature multi-effect distillation (MED), and reverse osmosis (RO). However, these technologies commonly have defects including high cost, high energy consumption, and low water production.
In recent years, forward osmosis (FO) seawater desalination technology has been widely examined. According to the principle of forward osmosis, the forward osmosis seawater desalination technology uses draw solutions (DS) with a high osmotic pressure to separate seawater and get fresh water. Although the forward osmosis seawater desalination technology has advantages over other technologies, such as its lower cost, lower energy consumption, and higher water production, appropriate draw solutions are still required to truly achieve a low-cost water production process.
Generally, polymer material has problems with solubility and high viscosity, so it is difficult to formulate a high-concentration solution using a polymer material. Thus, the osmotic pressure of the solution cannot be further increased. Although the low molecular weight polymer material has better solubility and can be formulated to form a high-concentration solution, the osmotic pressure is still not enough.
Many draw solutions may have a sufficiently high osmotic pressure; however, it is not suitable for practical promotion due to the high energy consumption. For example, while increasing the solubility or osmotic pressure of the draw solution by introducing carbon dioxide, additional processes of heating to 60° C. or higher are needed to remove carbon dioxide when recycling the draw solution. Therefore, higher energy consumption is needed. In addition, magnetic nanoparticles are reported to have been used as a draw solution and recycled by magnetic separation to realize the recirculation of the draw solution. However, in fact, the agglomerated magnetic particles are not easily dispersed again. Moreover, it is also difficult to remove the magnetic nanoparticles.
Therefore, a novel draw solution (extract) material is needed.